A typical method of making heat exchangers for a variety of gas- and oil-fired industrial or residential products is to bend a metal tube into a serpentine shape, thereby providing multiple passes. Gases heated by a burner at one end of the heat exchanger travel through the tube interior and exit the other end of the heat exchanger. While the hot flue gases are within the tube, heat is conducted through the metal walls of the tube and transferred to the air or other fluid media surrounding the tube, which raises its temperature. In order to achieve efficient heat transfer from the tubes, it is usually necessary to alter the flow of gases by reducing their velocity and/or promoting turbulence, mixing, and improved contact with the tube surface.